A bit of history is probably important to answer the question. Since March, I have lost 59 lbs by calorie restriction, a healthy diet, and cardiovascular exercise. I have not trained with weights other than carrying 2 lb hand weights and a 8 lb to 20 lb weight vest while walking. Furthermore, I have not used any weights during the time period in question. Last Sunday, I weighed in at 176.5 lbs, my lowest weight since high school. On Thursday, I weighed in at 182 lbs. Last night, I weighed in at 189 lbs. At each of these weigh ins, my percentage body fat was approximately 14% (+/- .5%).

My weight can easily swing five pounds in a 24 hour period of time depending on calories consumed, hydration levels, regularity of bowel movements, and the timing of my weigh ins. That said, these factors remained relatively constant throughout the week and do not, alone, account for the twelve pound swing in my weight.

Caloric Intake and Expenditure

My caloric intake for the week was approximately 15k calories. My caloric expenditure for the week, on top of my RMR, was 9k calories, as determined by a Polar F11 HRM worn during all exercise. This would, presumably, lead to weight loss. However, 2k of the additional caloric expenditure came on Thursday and another 3k of the additional caloric expenditure came on Saturday. Both days resulted in significant calorie deficits.

Theory, Being What It Is

It is my understanding that when the body is stressed through exercise and calorie deprivation at these levels that the body stores extra energy in some form, which can account for temporary weight gain when one might expect weight loss. This has happened several times to me and the weight melts off after a few days without any substantial change in my diet or exercise. However, such a change in my weight has never been so large before. Is this a plausible answer to my question? What is this mechanism called? Are there other factors I am not considering?

I would look at a medical explanation:
Are you female? If so, hormone swings can cause water weight gain and loss.
Do you have heart failure? Heart failure can cause fluid buildup and a 5 lb gain overnight. And you don't have to be older to have heart failure.
Do you have diabetes or some other metabolic condition?

Maybe you've hit a plateau like you are suggesting in your theory. If you have been checked out medically and everything is OK, consider adding weight training. An hour spread over a whole week is enough.posted by FergieBelle at 3:21 PM on August 20, 2007

Have you eaten a lot of salty food lately?

Is your scale broken?

How are you measuring your BF %? Body fat measurement is notoriously inaccurate, although if you at least measure yourself consistently then changes in percentage should be fairly accurate.

I don't know, that's an enormous amount of weight to fluctuate. Even if you did lift weights heavily, nobody builds pounds of muscle that quickly. Water weight is behind the majority of day-to-day fluctuations. This is most likely water weight you've added--though, why?

Thyroid problem perhaps?posted by Khalad at 3:24 PM on August 20, 2007

It's impossible that a 13-pound difference could be attributable to food or muscle. It's either fluid retention, a broken scale, or a combination of both.

Don't weight yourself every day. Just do what you're supposed to do, and the mirror (and your jeans) will tell you everything you need to know.posted by mpls2 at 3:55 PM on August 20, 2007

I had an alarming weight surge (by numbers, not so much dress size) in late July. Over a three day period, I gained nine pounds. It held for a day, then within two more days I was back down to my starting weight. It coincided with a heat wave and an increase in my salt intake, so I desperately prayed it was water weight - which of course it was. It's summer, it's hot, there's a good chance you're retaining fluid. Drink MORE water if you can manage it, and try and reduce your salt for just a couple of days and see if it has an effect.posted by annathea at 4:55 PM on August 20, 2007

Also, I disagree with the common wisdom that you shouldn't weigh yourself every day - it works much better for me to be familiar with my weight fluctuations, rather than weighing once a week and being subject to fluctuations that I haven't witnessed (for example, weighing 170 pounds last Friday, and 173 pounds this Friday, would be an uncomfortable "gain" for me, but if I'd been weighing daily I could gauge the trend and would know if my weight had been increasing daily or if it was just a blip).

I think it's different for everyone - but for me personally, weighing daily before I brush my teeth helps me add data to my personal telemetry.posted by annathea at 5:00 PM on August 20, 2007

Seconding possible circulatory problems. IANAD, but I was told that when patients with heart failure come into the ER, rapid weight gain was diagnostic. I'm also told that since that weight gain tends to be in your legs, a physical exam includes a little squeeze on (just above) a seated person's ankles to check for squishy water build up there. I can't really describe how it looks, and if you don't look at ankles on a regular basis you might not know if it looks different.

These are the canonical signs and symptoms, do you match any others? This is more technical. HF can be a chronic condition, not something that appears in minutes like a heart attack. I'm not saying this because it's the most likely cause, but because it's the most serious possible contender that would be good to rule out. If you get a regular checkup it is unlikely to be missed.posted by a robot made out of meat at 5:07 PM on August 20, 2007

But yeah, if it wasn't clear, water retention has lots of more benign causes. I'm your mefi alarmist.posted by a robot made out of meat at 5:08 PM on August 20, 2007

your metabolism might be going up as you get into better shape, so you might be retaining more water than usual.

also, you may be building muscle.

how do your clothes feel? 12 pounds of fat will make your pants feel tighter, 12 pounds of muscle not necessarily so.posted by thinkingwoman at 5:27 PM on August 20, 2007

Similar short-term gains have happened to me on my current eating plan. I have found that a refeed helps in such situations. Maybe it's the hormones going wacky, maybe the carbs are needed to fire off some process that's gumming up the metabolic works, but a day of serious complex-carb-heavy eating seems to do the trick.posted by backupjesus at 6:25 PM on August 20, 2007

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